The present invention relates to an acoustical signal analyzer for producing a sound spectrogram, and more particularly to a signal analyzer of this type which uses the method of zero-crossing analysis to produce a sound spectrogram.
In many areas of scientific research, there is a need to analyze the sounds produced by men, animals, and machines. For example, a study of the ultrasonic signals produced by bats is useful in understanding the process by which a bat can locate small fast-moving objects. This knowledge is being applied in the development of various types of apparatus including equipment to aid persons with visual handicaps.
A particularly useful type of acoustical signal analysis is the study of the frequency of sound signals produced over a period of time; that is, a spectral history of the sound. The output of instruments producing a spectral history of an acoustical signal is called a sound spectrogram, or a time versus frequency plot.
Various methods were used in the prior art to produce sound spectrograms. In one type of apparatus, analog audio signals were stored and subsequently analyzed by analog techniques using a sweeping bandpass filter or a bank of fixed bandpass filters. Alternatively, digital analysis was performed on the stored signal using a mathematical technique known as Fast Fourier Transformation (FFT). Although useful data were produced by such prior art apparatus, several problems were encountered. Since the signal was required to be stored and then subsequently analyzed, the output of data was often not as rapid as desired. Furthermore, such apparatus was large, complex, and expensive, and was thus more suitable for the laboratory than for field use.
A more economical method of producing a sound spectrogram involves the technique of zero-crossing analysis. In this technique, an analog AC audio signal alternating between positive and negative voltages is analyzed to measure the time period between successive crossings of zero voltage in the signal waveform. The time period between successive zero-crossings is the reciprocal of frequency; that is: frequency=1/period. Such time periods are much more easily measured than are frequencies of the analog signal being analyzed. By graphing the reciprocals of the time periods between successive zero-crossings of a signal of arbitrary duration, it is possible to obtain a spectral history of the principle or fundamental frequency component of the signal.
Signal analyzers using the technique of zero-crossing analysis can be constructed in small, lightweight, easily portable packages, thus providing instruments suitable for use in field research. Furthermore, since the accuracy of the measurements depends upon the stability of a clock signal produced by a crystal oscillator and not upon bulky and environmentally-dependent single frequency tuned circuits, zero crossing signal analyzers can be successfully operated under the adverse conditions often found in the field. Zero-crossing analysis is also inherently much faster than the prior art techniques of signal storage and after-the-fact analysis. Thus, a signal analyzer using zero-crossing techniques can perform signal analysis in substantially real time.
Unfortunately, zero-crossing signal analyzers of the prior art are susceptible to several problems. If a signal being analyzed includes a number of different frequencies, certain phase and amplitude relationships between the individual frequency components of the signal can cause known zero-crossing analyzers to produce inaccurate results. For example, one dominant frequency may sometimes completely mask the presence of another frequency. Under other conditions, two or more signals can combine to produce data indicating the presence of a spurious frequency not actually present in the waveform being analyzed.
It is therefore an objective of the present invention to provide apparatus for analyzing acoustical signals which incorporates the speed, portability, environmental insensitivity, and low cost of zero-crossing techniques, yet which will also provide the accuracy and data integrity of laboratory equipment.